1. Field of the Invention
Urea phosphate [CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4 ] is a dry, white crystalline material that melts at 243.5.degree. F. It contains 17.7 percent nitrogen and 19.6 percent phosphorus which is equivalent to 44.9 percent P.sub.2 O.sub.5. Urea phosphate is acidic (pH of 1 percent solution, 1.8), very soluble in water, and has a specific gravity of 1.759. The critical humidity of the material is 75 percent to 80 percent at 86.degree. F. and is similar to urea. It is well known that urea phosphate is a finished fertilizer. Agronomic tests of the material have shown that it is an efficient source of nitrogen and phosphate for plant growth. Numerous investigators of the prior art have proposed the use of urea phosphate as an intermediate in the production of solid and liquid fertilizers containing polyphosphate.
Wet-process orthophosphoric acid is prepared by the acidulation of phosphate rock (maInly fluoroapatite) with sulfuric acid and separating the resulting calcium sulfate from the acidulate. The wet-process acid contains impurities from the phosphate rock such as iron, aluminum, magnesium, fluorine and calcium. In addition, unless the phosphate rock is calcined before it is acidulated, the acid contains dissolved and suspended carbonaceous matter which imparts a black, opaque color to the acid. It is difficult to make high quality liquids from wet-process acid containing all of its impurities. Methods for separating impurities from wet-process acid have been described by numerous investigators. Most of these methods are solvent extraction processes. Use of solvent extraction techniques require large quantities of expensive solvents which must be recovered. Also, the solvents only partially eliminate metallic impurities and sulfate from the acids. In the urea phosphate crystallization process, most of the impurities are separated from the phosphoric acid by reacting the acid with urea and crystallizing the adduct, urea phosphate, from the solution; most of the impurities from the acid remain in the mother liquor. The reaction of urea and phosphoric acid to form urea phosphate is shown by the following equation: EQU CO(NH.sub.2).sub.2 +H.sub.3 PO.sub.4 .fwdarw.CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4
The urea phosphate is then crystallized from the solution. The urea-phosphoric acid reaction and the crystallization of urea phosphate are both exothermic. The enthalpy of producing crystalline urea phosphate from equimolar amounts of urea and phosphoric acid (75 percent H.sub.3 PO.sub.4) at 77.degree. F. is -44 calories per gram. To produce solutions containing polyphosphate, the solid urea phosphate first is heated, either by external means or by chemical heat of ammoniation, to form urea-ammonium polyphosphate melt. This melt then is used to produce relatively pure solution fertilizers. Procedures similar to those involved in this process have been reported in the literature.
Description of the Prior Art
Heretofore, the reaction of phosphoric acid with urea has been studied by a number of investigators, beginning, we believe, with the work described in a German Patent (No. 286,491) granted to Badische Anilin and Soda-Fabrik in 1914. In this work, one mole of urea was reacted with one mole of phosphoric acid (50 percent). The solution was cooled to crystallize urea phosphate and the crystals were separated from the mother liquor. The patent claims the use of urea phosphate as a fertilizer. Clarkson and Braham (U.S. Pat. No. 1,440,056) added one mole of urea/mole H.sub.3 PO.sub.4 to a solution containing 55-75 percent H.sub.3 PO.sub.4 and separated the resulting crystals of urea phosphate from the solution. According to the disclosure, evaporation of water from urea-phosphate solutions should be carried out at low temperatures since decomposition becomes rapid above 194.degree. F.
Somewhat more recently, Keens (British Pat. No. 1,149,924) proposed production of urea phosphate continuously in a vacuum crystallization process. The process utilized unconcentrated phosphoric acid and urea solution as feed material and involved heating and evaporation of the solution to yield a supersaturated solution of urea phosphate which was contacted with a suspension of growing urea phosphate crystals in mother liquor. The mother liquor was recycled to the feed solution. The final product crystals consisted of larger crystals which were more suitable for the manufacture of fertilizers. Mansfield (German Offen. Pat. No. 2,322,114) prepared urea phosphate by passing 90 percent H.sub.3 PO.sub.4 at 95.degree. F. and 90 percent urea at 239.degree. F. into a tube of 6 m length and 5.degree. inclination with countercurrent passage of warmed air. By this method, urea phosphate of &lt;0.7 percent moisture cohtent at .about.95.degree. F. was obtained. Greidinger and Cytter (German Offen. No. 2,429,030) describe a process for the manufacture of urea phosphate, useful as a fertilizer, by reaction of urea with anhydrous H.sub.3 PO.sub.4 optionally with the addition of Mg, CO, Fe, Zn, Cu or Mn trace elements.
In still another patent, Koebner, Edwards and Williams (British No. 1,191,635) prepared urea phosphate as an intermediate from wet-process H.sub.3 PO.sub.4 and treated the urea phosphate with an alkali metal or ammonium hydroxide or carbonate to produce orthophosphate and regenerate urea. The orthophosphate is separated as product and the urea is recycled. The reaction is preferably carried out in recycled mother liquor.
Methods of utilization of Vrea phosphate as the starting material for production of ammonium polyphosphate-type fertilizers have been proposed. These processes involve thermal decomposition of urea phosphate and utilize the condensing action of urea in urea phosphate to form polyphosphates. Among those is one method described by Theobald (German Patent No. 2,308,408) who proposes a two-stage process where urea phosphate is melted in the first stage and pyrolyzed into polyphosphate in the second stage.
In U.S. Pat. No. 3,713,802, Gittenait utilizes urea phosphate as the starting material for producing liquid and solid urea-ammonium polyphosphate. In the production of urea phosphate, unpurified wet-process phosphoric acid containing 30-60 percent P.sub.2 O.sub.5 is reacted directly with urea (solid or solution). Mother liquor is added to increase fluidity. The urea phosphate is crystallized out after one hour, for example, and removed from the mother liquor by centrifuging. Most of the mineral impurities accompanying the wet-process acid remain in the mother liquor. Stinson, Mann, and McCullough (U.S. Pat. No. 4,217,128) describe a process for production of urea-ammonium polyphosphates by pyrolysis of crystalline urea phosphate in one stage. Molten urea-ammonium polyphosphates that contain up to 95 percent of the phosphate as polyphosphate are obtained. These are then processed into high-analysis solid or liquid fertilizers. Addition of urea to the process to maintain a urea:biuret ratio of at least 16 prevents precipitation of biuret in the liquid fertilizers.
Kozo Fukuba (Japanese Pat. No. 49-8498) describes a method of production of a highly purified, water-insoluble ammonium polyphosphate. The process involves calcination of dried urea phosphate, under an ammonia atmosphere, at 390.degree. F. to 570.degree. F.
In the production of the intermediate urea phosphate by Fukuba, wet-process acid (29 percent P.sub.2 O.sub.5, as per example, produced by decomposition of Morrocan rock with sulfuric acid) was treated batchwise with caustic soda or sodium carbonate to remove some of the fluorine and silica; a large proportion of the organic materials in the acid are removed by using activated carbon and the acid is then concentrated to 45 to 55 percent by weight P.sub.2 O.sub.5 content. Urea is reacted with the concentrated wet-process acid and recycle mother liquor is added for fluidity. The mole ratio of urea to H.sub.3 PO.sub.4 in the feed is 0.9 to 1.5. The reaction to yield urea phosphate is carried out at about 120.degree. l F. to 160.degree. F. The reaction mixture is cooled to about 40.degree.0 F. to 86.degree. F. and crystalline urea phosphate is separated from the mother liquor. Fukuba's patent is characterized by the production of ammonium polyphosphate and the recycling of the mother liquor in the urea phosphate production system. Also, Fukuba discloses that addition of sulfuric acid increases yield of urea phosphate produced by the reaction of wet-process acid and urea.